Multiperiodic accelerator structures for linear particle accelerators

ABSTRACT

High efficiency linear accelerator structures comprising a succession of cylindrical resonant cavities which are accelerating cavities, and coupling annular cavities which are located at the periphery thereof, each of these annular cavities being coupled to two adjacent cylindrical cavities.

United States Patent [19] Tran [ MULTIPERIODIC ACCELERATOR STRUCTURES FOR LINEAR PARTICLE ACCELERATORS [75] Inventor:

[73] Assignee: C.G.R.-Mev., Paris, France [22] Filed: July 5, 1973 [21] Appl. No.: 376,584

Duc Tien Tran, Paris, France [30] Foreign Application Priority Data July 7, 1972 France 7224746 [52] US. Cl. 315/5.42; 3l5/3.5; 315/5.4l; 328/233 [51] Int. Cl. HOlj 25/10 [58] Field of Search 315/5.41, 5.42, 3.5 X, 315/3.5; 328/233 [56] References Cited UNITED STATES PATENTS 2,556,978 Pierce 3l5/5.42

[4 1 Sept. 16, 1975 2,582,186 1/1952 Willshaw 315/5.42 2,785,381 3/1957 Brown 333/73 W 3,221,204 11/1965 Hant et aL... 315/3.5 3,221,205 11/1965 Sensider 315/3.5 3,454,817 7/1969 Shively et a1. 3 15/5142 3,546,524 12/1970 Stark 315/5.42

Primary ExaminerArchie R. Borchelt Assistant Examiner-Saxfield Chatmon, Jr. Attorney, Agent, or Firm-Oblon, Fisher, Spivak, McClelland & Maier [57] ABSTRACT High efficiency linear accelerator structures comprising a succession of cylindrical resonant cavities which are accelerating cavities, and coupling annular cavities which are located at the periphery thereof, each of these annular cavities being coupled to two adjacent cylindrical cavities.

8 Claims, 16 Drawing Figures PAIENIEBS'EF isms SHEET 3 or 7 PATENTEU SEP I SW5 3, 906,300

PATENTEBSEP rams.

saw 5 ur 7 PATENTEBSEP 1's ms SHEET 7 OF MULTIPERIODIC ACCELERATOR STRUCTURES FOR LINEAR PARTICLE ACCELERATORS Multiperiodic structures in linear accelerators are FIGS. 15 and 16 respectively show resonance frequency curves of two biperiodic structures in accordance with the invention.

. The accelerator structure in accordance with the ingenerally constituted by groups of two or three resovention, schematically illustrated in FIG. 1, comprises nant cavities which are accelerating cavities; these aca succession of cylindrical cavities a,, 12 a [2 a;,, b;; celerating cavities are coupled with one another by a having a common axis X X substantially coincidental coupling cavity, each of such groups(two accelerating with the mean trajectory of the accelerated particle cavities and one coupling cavity) corresponding to one beam.

period of the electromagnetic wave which is created The Pairs of adjacent Cavities 1 and la 1 and 2 2 within this a celerati structure and b b and a a and b are preceded by an input It is important that, in such structures, the energy Cavity 0- stored in the accelerating cavities should be maximum Two adjacent cavities have a common Walls i. 1 and that the energy stored in the coupling cavities 5 2 "2 3 "at Perpendicular t0 the axis l 2- In the should be as low as i l bodiment shown by way of example, every wall m is It is an object of this invention to achieve this aim in provided with two holes and v for coupling purposes a particularly effective manner, while, in the same time, whilst h other walls have no Coupling apertures at providing a structure which is simple to manufacture, h cylindrical g i f are is readily adjustable and has an Operating frequency 20 spectively coupled, in pairs to annular cavities o 0 c which is not sensitive to minor machining or adjust- PQ coaxlauy at the peflphery of the cylmdnchl ment inaccuracies cavities (1,, b a b a 22 with which they are associ- In accordance with the invention, there is provided ated by means of couplmg holes 3 t3 a linear structure for a linear particle accelerator comformed m the Walls r espectwel.y commlon prising an input cavity and a plurality of successive ac- 25 an f i cavlty and to the pan of assoclated Cylmdn celcrating cylindrical resonant cavities, means for cou- Cal cavitles' the path. followed by the .electro pling said cylindrical resonant cavities to each other i which h been ed mm the first cavlty that is to say into the input cavity a,,, for example, by means for Said elctromagnenc engigy of a coupling loop B, is illustrated by a full line and the into said input cavity, said coupling means comprising reflected electromagnetic wave by a broken line.

a plurality of annular cavities coaxial with said Gyilfld-I'h The Shape and dimensions given to the first and last cal resonant cavltfes and fi at h penphenes cylindrical cavities a and b,, of the structure, and the thereof, each of said annular cavities being coupled to Shape and dimensions given tothe annular Cavities of two adjacent cylindrical resonant Cavities coupling cavities c are so selected that, as known, in the For the better understanding of the invention and to 3 Standing wave operation the Components of incident Show how the Same Way be Carried into effect refer and reflected waves add to each other in the cylindrical ehcc will be made m the drawings aCCQmpahyihg the cavities and cancel one another out in the annular caviehsuihg description in which: ties. The vectoral diagram of FIG. 2 illustrates the FIG. 1 schematically illustrates a linear structure in ha hift 2 /3 hi h i t between th iti accordance with the invention, 40 12,, of a 2'n'mode" triperiodic structure.

FIG. 2 illustrates a vector diagram indicating the FIG. 3 schematically illustrates an example of an anphase shift in the incident wave and reflected wave in nular cavity and FIG. 4 shows its equivalent circuit diaone set of three associated cavities (two accelerating gram. cavities and one coupling avity Designating by r and r+ A) the QUIET radius Of the an- FIGS. 3 and 4 schematically illustrate an example of l r Cavity Ci, by AZ the length of the terminal induca coupling cavity in accordance with the invention. and five Portions S1 and 2 0f the annular Cavities i y 2 the equivalent circuit diagram of said cavity, AZ of Central inductive portions 3 y 1 the length of F IG. 5 illustrates in detail the elements of a structure the Tia-entrant Portions 1 and 2 these Portions h 2, according to the invention S;,, R,, R being respectively illustrated in the equiva- FIGS. 6 and 7 respectively illustrate a triperiodic lent Circuit diagram of 4 as ihducators 1 L2 and structure according to the invention and in a somewhat and the P I 1 and 2 the Value of the inducsimplified manner, the arrangement of the coupling ap- Hons L1 and L2 gweh by: ertures between the different cavities of this structure,

FIGS. 8, 9 and 10 respectively illustrate another embodiment of an accelerator structure according to the L] L): "4AM: invention, and two tuning systems therefor,

FIGS. 11 to 14 illustrate still other embodiments of i I ,a structure according to the invention and, whilst that of the capacitances C and C is given by:

C1 C 21750.}. 2Uo(r +Ar) Log 1 dz where d is the width of the capactivie space formed by the reentrant portions.

If the inductance of the intermediate inductive portion L is equal to 2 L,, the resonance frequency of the annular cavity is given by:

In the example chosen.f= 3 Ghl. andl= 7.5 mm Ar= mm :1 2.5 mm

r 40 mm A: 10 mm FIG. 6 illustrates, in longitudinal section, an example of a triperiodic structure in accordance with the invention. In this example, the accelerator structure is produced by stacking, into a cylindrical sleeve 13, elements e and e which are solids of revolution, such as shown in FIG. 5. Each element e, has substantially the shape of a circular plate rn exhibiting a central portion 2 having an increase thickness through which a hole 3 extends. Each element e is in the form of a cylindar the lateral wall of which is constituted with an embattle ring 6. This cylinder is provided with a circular wall n comprising a central portion 4 having an increased thickness and through which a hole 5 extends. The elements e are furthermore provided with apertures u and v formed in their circular plate m and the rings 6 constituting the lateral walls of the elements 6 contain two apertures .s and t disposed symmetrically at either side of the circular plate n. The rings 6 of the elements e spectively associated with each two adjacent cavities a,- and b,- to tune these cavities c,-, a,- and b,-.

The structures described hereinbefore are of the triperiodic type but biperiodic structures can be produced in a similar way. FIG. 11 illustrates in longitudinal section a biperiodic structure operating in the 'n' mode. The length of the cylindrical accelerator cavity is in this case equal to BA/Z, being the reduced velocity of the particles propagating through the cylindrical cavities and A the operating wavelength of the structure. Such a structure is particularly suitable because it is constituted by identical elements 30, which build up cylindrical cavities d d d The coupling between two adjacent cylindrical cavities d d d d is effected solely through the medium of the annular cavities f,,f by means of coupling apertures s s 5 components of the electric field H of the electromagexhibit two shoulders 11 and 12 between which accomodate the plates m of the elements 0 The elements e, and e are assembled together in the manner shown in FIG. 6, within a cylindrical sleeve 13 thus forming cylindrical cavities a b (1 12 (1 b and annular cavities 0,, c c The coupling between the cavities b and a b and a is effected through apertures 11,, v n v. formed in the circular plates In. To avoid direct coupling between the cavities c and a and a the coupling apertures u v u v are arranged in such a fashion that they are staggered in relation to the apertures s 2,, s 2 as shown in the cut-away perspective view of FIG. 7.

The cavities a and b,, at the ends of the structure are identical to one another but differ slightly from the cavities a a a b b Their dimensions are such that at the resonance frequency of the accelerating cavities a a b b when the latter are operating in the 211" mode for example, they subject the reflected wave to a phase shift of 7r/2 in relation to the input wave so that within the accelerating structure a standing wave situation is created in which the electromagnetic field is cancelled in all the annular coupling cavities so that optimum efficiency on the part of the structure is ensured.

FIG. 8 illustrates embodiment of a triperiodic structure in accordance with the invention. The elements 20 and 21 are assembled by means of rods which are, for example, four and are 90 angularly spaced. These rods extend through tubular passages 22 longitudinally disposed at the periphery of the elements 20 and 21. At least one adjustable tuning plunger 23 is associated with each annular coupling cavity 6,- (FIG. 9), and at least two adjustable tuning plungers 24 and 25 are renetic wave being distributed within the structure in the manner indicated in FIG. 12.

This Zmnode biperiodic structure is better suited to accelerators of relatively low energy whereas mnode biperiodic structures and 21rmode triperiodic structures are better suited to high energy accelerators. In other words, the efficiency of a structure is better if the length of the accelerator cavity is substantially equal to their radius; however this radius depends essentially upon the operating wavelength A. Thus, the fact that the length of a cell of a 11' mode biperiodic structure is ,BA (instead of BA/Z as in the case of 11' mode biperiodic structures or 211' mode" triperiodic structures), favours the acceleration of particles of relatively low velocity.

The structures in accordance with the invention have been described by way of non-[imitative examples and their characteristics generally depend on the selected dimensions of the coupling apertures, on their location and number. By a suitable selection of the parameters it is possible to obtain the desired predetermined operating pass band.

It is also possible to improve the operation of a Tr/2 mode biperiodic structure having annular coupling cavities c c c,, with two re-entrant portions such as illustrated in FIG. 6, by arranging the coupling apertures s and t not at the ends of the annular cavity c 0,, as in FIG. 6 but in the central part of said annular cavity as shown in FIG. 13.

Asa matter of fact, the resonance frequency of the system formed by the accelerating cavities a,-, b,- and coupling cavities; 0, highly depends upon the coupling factor, and therefore upon the dimension of the coupling apertures s, I when said apertures s, I are arranged at the ends of the reentrant section of the annular cavity c,-. It is possible to remedy this drawback by arranging the coupling apertures S, t at the centre of the annular cavity 0, (FIG. 13'), as mentioned before.

Apertures .s' and I are therefore formed obliquely in the central zone of the wall 2 (FIG. 13) common to the accelerating cavities a,-, b,- and to the coupling cavity 0,- associated therewith, these apertures s and t, which re spectively couple the annular cavity c,- with the accelerating cavities a,- and b,-, being arranged on different radii making an angle with one another in order to avoid direct coupling between the two apertures s, t. The dimensions of these apertures s and t arranged in the central zone of the annular cavity 0,- are not critical in so far as the resonance frequency of the accelerating structure is concerned. This makes it possible to adjust separately the frequency of the structure and the coupling between the cavities.

FIGS. and 16 respectively illustrate, in the case of a biperiodic structure operating in the 1r/2 mode, the variation of the resonance frequency of the structure as a function of the dimensions d d d of the coupling apertures s, t when these apertures are arranged at the ends of the annular cavity c,- (FIG. 15) and in the case where the apertures are arranged at the centre of said annular cavity c,-( FIG. 16).

FIG. 14 illustrates a biperiodic 211' mode" structure, the annular cavity c,- of which has three re-entrant portions. The coupling apertures s and t are in this case arranged at either side of the central re-entrant portion and in different planes, in order to avoid direct coupling between the apertures s and t.

The particle accelerator structures in accordance with the invention can advantageously be utilised in linear electron or proton accelerators.

What we claim is:

l. A linear structure for a linear accelerator comprising an input cavity and a plurality of successive pairs of accelerating cylindrical resonant cavities, means ineluding coupling apertures for coupling said pairs of cavities, means for coupling said cavities forming each said pair to each other comprising a plurality of annular cavities each coupled through apertures to one of said pairs of cavities, said annular coupling cavities arranged coaxially with said cylindrical resonant cavities and located at the periphery thereof; and means for feeding electromagnetic energy into said input cavity.

2. A linear structure as claimed in claim 1, wherein said annular cavities have a section of re-entrant profile type.

3. A linear structure as claimed in claim 2, wherein said structure is biperiodic.

4. A linear structure as claimed in claim 3, wherein every two adjacent cylindrical cavities are coupled to each other by means one of said annular cavities, said annular cavities having a re-entrant section, and being provided with coupling apertures (s, t).

5. A linear structure as claimed in claim 4, wherein said apertures (s, t) are arranged at the ends of said reentrant section, said section having two re-entrant portions.

6. A linear structure as claimed in claim 4, wherein said apertures (s, t) are located in the central portion of said re-entrant section, said section having two reentrant portions.

7. A structure as claimed in claim 2, wherein said structure is triperiodic.

8. A linear structure as claimed in claim 7, wherein adjacent pairs of cylindrical cavities are coupled through apertures formed in a transverse common wall of said adjacent pairs of cylindrical cavities. 

1. A linear structure for a linear accelerator comprising an input cavity and a plurality of successive pairs of accelerating cylindrical resonant cavities, means including coupling apertures for coupling said pairs of cavities, means for coupling said cavities forming each said pair to each other comprising a plurality of annular cavities each coupled through apertures to one of said pairs of cavities, said annular coupling cavities arranged coaxially with said cylindrical resonant cavities and located at the periphery thereof; and means for feeding electromagnetic energy into said input cavity.
 2. A linear structure as claimed in claim 1, wherein said annular cavities have a section of re-entrant profile type.
 3. A linear structure as claimed in claim 2, wherein said structure is biperiodic.
 4. A linear structure as claimed in claim 3, wherein every two adjacent cylindrical cavities are coupled to each other by means one of said annular cavities, said annular cavities having a re-entrant section, and being provided with coupling apertures (s, t).
 5. A linear structure as claimed in claim 4, wherein said apertures (s, t) are arranged at the ends of said re-entrant section, said section having two re-entrant portions.
 6. A linear structure as claimed in claim 4, wherein said apertures (s, t) are located in the central portion of said re-entrant section, said section having two re-entrant portions.
 7. A structure as claimed in claim 2, wherein said structure is triperiodic.
 8. A linear structure as claimed in claim 7, wherein adjacent pairs of cylindrical cavities are coupled through apertures formed in a transverse common wall of said adjacent pairs of cylindrical cavities. 